This invention relates to a method for manufacturing a magnetic disk substrate made of glass-ceramic having an improved surface characteristic after polishing.
There is an increasing demand for a magnetic disk as an external storage medium of an office computer and a personal computer and various developments have been made for improving characteristics of a magnetic disk. A substrate for a magnetic disk is generally required to have the following characteristics:
(1) In the CSS (contact-start-stop) characteristic (hereinafter referred to as "CSS characteristic") of a magnetic disk, if the disk has a smooth surface having surface roughness (Ra) below 15.ANG., sticking between the head and the disk occurs due to increase in contact resistance caused by high rotation of the disk and, for this reason, the surface roughness (Ra) should be larger than 15.ANG.. Further, if the surface of the disk is a rough one having the surface roughness (Ra) exceeding 50.ANG., damage of the head or the disk tends to take place and, for this reason, the surface roughness (Ra) should not exceed 50.ANG..
(2) There is tendency to reducing glide height (i.e., interval between the head and surface of the disk) for improving recording density of the magnetic disk and, for this reason, the surface of the disk should be flat and smooth to such extent that a desired glide height can be achieved.
(3) The magnetic disk substrate should be free from crystal anisotropy and other defects and should have fine and uniform texture.
(4) The substrate should have mechanical strength and hardness which are sufficient for standing high speed rotation of the disk and contact with the head.
(5) The substrate should not include Na.sub.2 O ingredient in its material because if the substrate includes the Na.sub.2 O ingredient, a Na ion is diffused in the substrate during a film forming process with resulting deterioration in the characteristics of the substrate.
(6) The substrate should have chemical durability for resisting rinsing and etching with various chemicals.
An aluminum alloy has generally been used as a magnetic disk substrate in the past. The aluminum alloy however tends to develop undesirable projections or spot-like projections and depressions on the surface of the surface of the substrate during a polishing process due to various defects existing inherently in the material with resulting deficiency in its flatness and surface roughness. The aluminum alloy therefore cannot satisfy the current demand for high density recording required for coping with increase in the amount of information.
Known also in the art are various types of magnetic disk substrates made of a chemically reinforced glass which have been developed for eliminating the above described disadvantages of the magnetic disk substrate made of aluminum alloy. These magnetic disk substrates made of a chemically reinforced glass however have the following disadvantages:
(1) Since polishing of the substrate is made after the chemical reinforcing processing, the reinforced layer tends to become instable as the thickness of the magnetic disk is decreased.
(2) The substrate must be subjected to a mechanical or chemical texturing in order to improve the contact-start-stop characteristic and this requirement makes it difficult to manufacture magnetic disks in a large scale and at a low cost.
(3) Since the substrate includes Na.sub.2 O as an essential ingredient, the film forming characteristic is deteriorated and, as a result, a surface coating process becomes necessary. Further, for eliminating the requirement for the texturing for improving the contact-start-stop characteristic in the chemically reinforced glass or glass-ceramics, a process has recently been developed for making the surface of the substrate rough. This process however is also insufficient for producing magnetic disks in a large scale and at a low cost.
Some glass-ceramics are known as materials satisfying some of the above described requirements. For example, Japanese Patent Applcation Laid-open No. Sho 60-229234 discloses a SiO.sub.2 --Al.sub.2 O.sub.3 --Li.sub.2 O system glass-ceramic which includes alpha-quartz solid solution or beta-spodumene solid-solution as a main crystal phase and consists of crystal grains having a diameter of about 0.1-1.0 .mu.m. Japanese Patent Application Laid-open No. Sho 62-72547 discloses a SiO.sub.2 --Li.sub.2 O system glass-ceramic which includes, as main crystal phases, granular lithium disilicate having a grain diameter of about 0.3-1.5 .mu.m and granular lithium metasilicate having a grain diameter of about 0.3-0.5 m. U.S. Pat. No. 3,231,456 discloses a Sio.sub.2 --Li.sub.2 O--P.sub.2 O.sub.5 --MgO system glass-ceramic added with CuO and SnO ingredients which includes lithium disilicate and as a main crystal phase and alpha-quartz as a subsidiary crystal phase.
U.S. Pat. No. 3,977,857 discloses a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 --(Na.sub.2 O+K.sub.2 O) system glass-ceramic which is suitable as a material which adheres directly to a metal material. The U.S. Patent describes that the main crystal phase of this glass-ceramic is Li.sub.2 O.multidot.2SiO.sub.2.
Japanese Patent Application Laid-open No. Sho 63-210039 discloses a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 system glass-ceramic which is suitable for use as a magnetic disk substrate. This publication describes that the main crystal phases of this glass-ceramic are Li.sub.2 O.multidot.2SiO.sub.2 and alpha-cristobalite.
In the prior art Li.sub.2 O--SiO.sub.2 system glass-ceramics (Japanese Patent Application Laid-open No. Sho 62-72547, U.S. Pat. No. 3,232,456, U.S. Pat. No. 3,977,857 and Japanese Patent Application Laid-open No. Sho 63-210039, crystal phases grown are Li.sub.2 O.multidot.2SiO.sub.2 as a main crystal phase and a small amount of SiO.sub.2 ( alpha-cristobalite or alpha-quartz). In these prior art glass-ceramics, it is the Li.sub.2 O.multidot.2SiO.sub.2 crystal phase as the main crystal phase and not the alpha-quartz or alpha-cristobalite crystal phase that performs a principal function. These prior art glass-ceramics cannot provide, as a surface characteristic existing inherently in the glass-ceramics after polishing, the surface roughness of 15.ANG.-50.ANG. which is necessary for the CSS characteristic of a magnetic disk. For this reason, for improving the CSS characteristic required for a magnetic disk substrate, some texturing processing is required after the polishing process for making the surface of the glass-ceramic rough. This prevents a large scale production of a magnetic disk having the above described characteristics at a low cost.
For overcoming the above described disadvantages of the prior art magnetic disk substrates, the assignee of the present invention has found, as a result of studies and experiments made for the purpose of eliminating the above described disadvantages of the prior art magnetic disk substrates and providing a magnetic disk substrate made of a glass-ceramic which has an excellent surface characteristic after polishing by controlling the crystal structure and crystal grains of crystal grown in the glass-ceramic, and providing also a method for manufacturing the magnetic disk substrate, that a glass-ceramic obtained by subjecting a SiO.sub.2 --Li.sub.2 O--P.sub.2 O.sub.5 system glass including MgO as an essential ingredient to a heat treatment under a strictly limited temperature range has a microstructure in which alpha-quartz (alpha-SiO.sub.2) in the form of aggregated globular particles is randomly grown in a uniform crystal phase of lithium disilicate(Li.sub.2 O.multidot.2SiO.sub.2) and that, since this microstructure consists of the lithium disilicate phase which is mechanically and chemically instable and the alpha-quartz phase which is mechanically and chemically stable, projections and depressions are produced on the surface of the glass-ceramic substrate after polishing owing to mechanical and chemical actions produced by lapping and polishing processes, and that a magnetic disk substrate having an excellent surface characteristic after polishing can be obtained by controlling the diameter of the aggregated particles of alpha-quartz.
This magnetic disk substrate is one described in the specification of U.S. patent application Ser. No. 08/307,889 filed on Sep. 16, 1994 which is made of a glass-ceramic having lithium disilicate (Li.sub.2 O.multidot.2SiO.sub.2) and alpha-quartz (alpha-SiO.sub.2) as predominant crystal phases, in which grown crystal grains of alpha-quartz (alpha-SiO.sub.2) have a globular grain structure each consisting of aggregated particles, the globular grain have a diameter within a range of 0.3 .mu.m-3.0 .mu.m, and the surface roughness (Ra) of the surface the substrate after polishing is within a range of 15.ANG.-50.ANG.. The glass-ceramic substrate can be obtained by subjecting to a heat treatment a base glass consisting in weight percent of 65-83% SiO.sub.2, 8-13% Li.sub.2 O, 0-7% K.sub.2 O, 0.5-5% MgO+ZnO+PbO in which 0.5-5% MgO, 0-5% ZnO, 0-5% PbO, 1-4% P.sub.2 O.sub.5, 0-7% Al.sub.2 O.sub.3 and 0-2% As.sub.2 O.sub.3 +Sb.sub.2 O.sub.3.
In the CSS characteristic of a magnetic disk, the surface roughness (Ra) of the disk should be within the range of 15.ANG.-50.ANG. and the above described glass-ceramic developed by the assignee of the present invention can achieve this surface roughness of 15.ANG.-50.ANG. without the necessity for a mechanical or chemical texturing. However, decrease in the glide height necessary for increasing recording density of a magnetic disk cannot be automatically achieved by achieving the surface roughness of 15.ANG.-50.ANG..
The glide height is determined on the basis of surface roughness (Ra) and surface waviness (Wa) of the magnetic disk substrate. The glide height is lower as a value of each characteristic (Ra or Wa) is smaller. As an index indicating a limit value of glide height, GABP (glide avalanche break point, its unit being "microinch" (.mu.")) is currently employed. GABP presently required for a magnetic disk substrate is in the order between 1.0 .mu." to 1.5 .mu." but a smaller glide height will be required as a demand for a high density recording becomes stronger.
Polishing of glass-ceramics has been generally carried out by using a polishing material such as cerium oxide having a grain diameter of 1 .mu.m to 2 .mu.m which is generally used for polishing optical glasses. In the case of the above described glass-ceramic developed by the assignee of the present invention, however, glide height of GABP exceeding 0.8 .mu." only can be achieved by this conventional polishing method, though the surface roughness within the range of 15.ANG.-50.ANG. can be achieved. Thus, it has been found that it is difficult to achieve a glide height of GABP 0.8 .mu." or below by the conventional method and therefore the conventional polishing method cannot cope with the future requirement for a reduced glide height.
The invention has been made to overcome the above described problem arising in achieving by polishing of a glass-ceramic a very small glide height required for achieving a high density recording.
It is, therefore, an object of the present invention to provide a method for manufacturing a magnetic disk substrate made of glass-ceramic which can satisfy the requirement for reduction in the glide height which will occur in future.